1. Field of the Invention
The invention relates to a radar system, a signal processing apparatus for the radar system, and a signal processing method for the radar system.
2. Description of the Related Art
It is known that a reception power from a target existing at a distance R is proportional to a radar cross section RCS of the target and inversely proportional to the fourth power of the distance to the target. Therefore, the RCS of the target can be calculated with the power received from the target and the distance to the target. An RCS value is an index of easiness of reflection of a radio wave. It is generally known that the larger the RCS value is, the larger the target is, although it is influenced by a shape and a material. That is, the calculated RCS value is one of important indices to decide a degree of importance and a degree of danger of the target. Particularly, it is important information when the system is applied to pre-crash safety or the like using a vehicle-mounted radar system.
However, there is such a problem that when a reception signal from the target exceeds a dynamic (input voltage) range of an A/D converter, the reception signal is clamped, harmonics appear in an FFT waveform, inherent target peak strength of fundamental wave appears to be small, and the RCS value cannot be properly calculated. Even if the harmonics appear, they do not (fundamentally) affect the measurement of the distance to the target and a relative velocity. The problem is that the peak strength seems to be small and it also makes the RCS value to seem small. Such a problem can occur when the RCS of the target is large or when the distance to the target is short. To solve such a problem, for example, in JP-A-2-27283, there is disclosed a system in which an AGC (Auto Gain Controller) circuit is provided in an amplifier disposed between a receiving antenna and an A/D converter and when a large reception signal is inputted, an amplification gain is suppressed, thereby preventing an input signal from exceeding the dynamic (input voltage) range of the A/D converter.
In the case of using the above conventional system, when a large reflection power from a certain target is received, the gain is suppressed by the AGC so that the input signal does not exceed the dynamic (input voltage) range of the A/D converter. However, there is such a problem that since the gain of the whole reception wave decreases, when a target peak of the small reception power exists at the same time, a sufficient amplification degree cannot be obtained, the target peak is below a noise floor, and there is a possibility that the target peak cannot be detected.